With rates skyrocketing over the last decade, weight loss (bariatric) surgery has become the second most common abdominal operation in the United States. Although bariatric surgery has become safer over time, venous thromboembolism (VTE) remains a major cause of potentially preventable morbidity and mortality. Prophylactic strategies, including perioperative anticoagulation and inferior vena cava (IVC) filter placement, may reduce the risk of VTE and its sequelae. However, these treatments are themselves associated with risks of complications and are very costly. In the absence of good evidence regarding optimal VTE prophylaxis, practices vary widely among surgeons and hospitals. Among the 23 bariatric surgery programs in Michigan, for example, the use of low-molecular weight heparin following discharge after gastric bypass varied from 0% to 96%; the use of preoperative IVC filter placement varied from 0-40%. The most aggressive practice styles do not necessarily translate to safer surgery. For example, rates of the most serious complications (resulting in death or permanent disability) are 3-times higher at the 5 Michigan hospitals that place IVC filters in more than 10% of their patients. Unwanted practice variation also has obvious implications for health care costs. Payments associated with IVC filter placement at the 5 outlier hospitals exceed several million dollars annually. The goals of this project are to reduce variation in VTE prophylaxis among hospitals performing bariatric surgery in Michigan, with the ultimate goal of improving outcomes and reducing costs. The study will be based on the prospective clinical registry and quality improvement infrastructure of the Michigan Bariatric Surgery Collaborative (MBSC), a statewide consortium involving all hospitals and surgeons performing bariatric surgery in Michigan. In terms of approach, our project will build off of templates for data-driven, collaborative quality improvement established by the Keystone ICU Project, a joint endeavor between patient safety experts at Johns Hopkins University and the Michigan Health and Hospital Association that has been extraordinarily successful in reducing rates of catheter sepsis and other intensive care complications in Michigan. Venous thromboembolism (VTE) is the most common preventable cause of hospital death. In the absence of good evidence regarding optimal VTE prohylaxis in bariatric surgery, practices and outcomes vary widely. The goals of this project are to reduce variation in VTE propylaxis among hospitals performing bariatric surgery in Michigan, with the ultimate goal of improving outcomes and reducing costs.